recommended reading

IRC snippets

JIT currently only is invoked at the start of loops. Basically, JIT works by converting Octave's AST to the LLVM Internal Representation (IR). This code is in libinterp/interp-core/pt-jit.cc. This is achieved by constructing an intermediate IR, which is used for type inference, then converting that intermediate IR to LLVM's IR.
If the conversion fails at any stage, we give up and use the interpreter. Normally, this is done by throwing a jit_fail_exception. The reason why we can't go directly to the LLVM IR, is because Octave's AST does not contain any type information, but LLVM's IR requires it. For example, A = B+C. If B and C are matrices, this has as a drastically different meaning than if B and C are scalars. We use the current types of variables to determine the types of the rest of the variables in the loop.